When DOD inkjet is considered, two main groups can be discerned: thermal inkjet and piezo inkjet.
With thermal inkjet technology, tiny resistors rapidly heat a thin layer of liquid ink. The heated ink causes a vapour bubble to be formed, expelling or ejecting drops of ink through nozzles and placing them precisely on a surface to form text or images. As the bubble collapses, it creates a vacuum that pulls in fresh ink. This process is repeated thousands of times per second. With thermal inkjet technology, water-based inks are used.
Piezoelectric printing technology—commonly called piezo—pumps ink through nozzles using pressure, like a squirt gun. A piezo crystal used as a very precise pump places ink onto the printing medium. A wide range of ink formulations (solvent, water, UV) may be used.
In heads used for high resolution printing, nozzles are located close to each other. Different nozzles next to each other suffer from cross-talk, both thermal cross-talk and mechanical cross-talk. The most severe form of cross-talk is mechanical cross-talk generated by using a common wall or shared wall between two nozzles, as explained hereinafter.
A number of different piezo concepts exist.
A typical concept, as described in U.S. Pat. No. 4,887,100, WO 96/10488, WO 97/04963 and WO 99/12738, all herein incorporated by reference in their entirety for background information, uses so called shared walls. The pressure chambers containing the ink are next to each other, while their dividing walls are the actuators.
Because an actuator is always shared by two channels, it is not possible to jet a drop out of two neighbouring channels at the same time. In WO 96/10488 is described that the nozzles are divided in three interlaced groups (A, B, C). Neighbouring nozzles are fired in a sequence ABC. Two solutions are possible to print dots on a straight line.
A first solution uses a complete nozzle array under a certain angle. By doing this, the resolution is increased, and by using the right fast scan speed, dots fired in a sequence A, B, C are on a straight line.
A second solution uses a head perpendicular to the fast scan direction, in which the A, B, and C nozzles are staggered in the is fast scan direction. Printing of a line of pixels is divided into three cycles. In the first cycle, the dividing walls to either side of the A channels are driven (if ink is to be ejected from them—depending on the image to be printed) with a pulsed signal. In the second cycle, the dividing walls to either side of the B channels are driven (if ink is to be ejected from them—depending on the image to be printed) with a pulsed signal. In the third cycle, the dividing walls to either side of the C channels are driven (if ink is to be ejected from them—depending on the image to be printed) with a pulsed signal. The pressure pulses developed in the channels that are not included in the current cycle are not larger than ½ of those in the channels that are intended to eject ink. The printing apparatus is arranged so that such pulses with ½ magnitude do not cause ink ejection.
A drawback of this concept is that, once the firing frequency is defined, only one fast scan speed can be used to print ABC dots on a straight line, as explained hereinafter. In the fast scan direction, the head will e.g. print each {fraction (1/360)}-inch.
FIG. 1 shows a piezo printhead 10 according to the prior art, having nozzles 12 which are divided into three sets, called a set of A nozzles, a set of B nozzles and a set of C nozzles, each set intended to be fired during different firing cycles. The different sets of nozzles are staggered with respect to each other over a stagger distance D1 in the fast scan direction. If the nozzles are divided in groups G of three, every first nozzle is part of the set of A nozzles, every second nozzle is part of the set of B nozzles and every third nozzle is part of the set of C nozzles. All nozzles in one set A, B, C are positioned on a straight line in the slow scan direction S, which lines are located at the stagger distance D1 with respect to each other in the fast scan direction F.
As an example, printhead 10 is considered to be a type 360 head. This means that the printhead 10 is provided for printing 360 dpi (=pixels per inch) in the fast scan direction F. In this type 360 printhead 10, the distance D1 between nozzles 12 in the fast scan direction F is {fraction (1/360)} inch/3=70.56 μm/3=23.52 μm.
If the firing frequency is 12.4 kHz, meaning that every set A, B, C of nozzles can be fired every 80.65 μs, the speed of the printhead 10 in the fast scan direction F is {fraction (1/360)} inch*12.4 kHz=0.875 m/s. The nozzles 12 are fired in an ABC sequence, with the A nozzles at the leading edge of the printhead 10 in the fast scan direction F.
The cycle frequency is 12.4 kHz*3=37.2 kHz. Or formulated in another way: the set of B nozzles fires 26.88 μs after the set of A nozzles, and the set of C nozzles fires 53.76 μs after the set of A nozzles. After 80.65 μs, the set of A nozzles fires again.
When it would be desired to keep the same firing frequency, but to print a 180*180 dpi image with the 360 type printhead of the example given above, the printhead speed should theoretically double to 1.750 m/s. In the above case of printing a 180*180 dpi image with a 360 type printhead, where the printhead speed must double to 1.750 m/s, the delays for firing B and C need to be shorter to make sure that dots are printed on the same line. Nozzle set B has to be fired 13.44 μs after nozzle set A, and nozzle set C 26.88 μs after nozzle set A. These firing frequencies are too close one to the other, and therefore a 360 type printhead cannot be used to print a 180*180 dpi image.
When it would be desired, on the other hand, to print a 720*720 dpi image with the 360 type printhead, the firing delay between the set of A nozzles, set of B nozzles and set of C nozzles increases to 53.76 μs. As, however, after 80.65 μs the set of A nozzles has to fire again, there is not enough time left to fire the set of C nozzles, and therefore a 360 type printhead cannot be used to print a 720*720 dpi image neither.
It is an object of the present invention to provide a method and device for printing, with one type of printhead, with a resolution which is different from the design resolution of the type of printhead used.
It is an object of the present invention to provide a method and device for printing, with one type of printhead, a variety of resolutions.